The classical form of α1-antitrypsin (“AT”) deficiency is an autosomal co-dominant disorder that affects approximately 1 in 2000 live births (25). It is caused by a point mutation that alters the folding of an abundant liver-derived plasma glycoprotein during biogenesis and also renders it prone to polymerization (43). In addition to the formation of insoluble aggregates in the ER of liver cells, there is an 85-90% reduction in circulating levels of AT, the pre-dominant physiologic inhibitor of neutrophil elastase. Individuals who are homozygous for the mutant allele are susceptible to premature development of chronic obstructive pulmonary disease. Pulmonary involvement is believed to be caused by a loss-of-function mechanism, as lack of AT in the lung permits elastase to slowly destroy the pulmonary connective tissue matrix (44).
AT deficiency is the most common genetic cause of liver disease in children and also causes liver disease and hepatocellular carcinoma in adults. In contrast to pulmonary involvement, liver inflammation and carcinogenesis are believed to be caused by a gain-of-toxic function mechanism. This is most clearly demonstrated by introducing the mutant human ATZ allele as transgene into genetically engineered mice (45, 11). Insoluble aggregates in hepatocytes, hepatic inflammation and carcinogenesis evolve even though the endogenous anti-elastases of the transgenic mouse are intact.
Cohort studies from an unbiased Swedish newborn screening program have shown that only 8-10% of the affected homozygous population develop clinically significant liver disease through the first 30 years of life (26). This has led to the concept that genetic and/or environmental modifiers determine whether an affected homozygote is susceptible to, or protected from, liver disease. Furthermore, it has led to consideration of two general explanations for the effects of such modifiers: variation in the function of intracellular degradative mechanisms and/or variation in the signal transduction pathways that are activated to protect the cell from protein mislocalization and/or aggregation.
Studies in this area have so far indicated that the proteasome is responsible for degrading soluble forms of ATZ (29, 46) and that maeroautophagy is specialized for disposal of the insoluble polymers/aggregates that accumulate in the ER (30, 47). In terms of cellular response pathways, it is thought that accumulation of ATZ activates NFκB and autophagy but not the unfolded protein response (1, 16).
Polymerization of protein is associated with a number of other disorders. Among these is Alzheimer's Disease (“AD”), a disorder which affects four million people in the United States and has an incidence estimated at 1 in 68 individuals. As such, AD is the most common form of age-dependent neurodegeneration. Most cases are recognized by the sporadic onset of dementia during the seventh decade of life while the less common, mutation-linked familial cases cause dementia that is recognized by the fifth decade. AD is associated with the accumulation of aggregation-prone peptides in the brain, especially amyloid-β (“Aβ”) peptides, but hyperphosphorylated tau proteins also contribute to the tangles and plaques that constitute the histological hallmarks of the disease.
AD is thought to be caused by a gain-of-toxic function mechanism that is triggered by the accumulation of aggregated Aβ and tau and worsened by aging (36). Recent studies have shown that the prevalence of autophagosomes is increased in dystrophic neurons of the AD brain, a finding that is recapitulated in mouse models of the disease (37). Most of the evidence suggests that autophagy plays a role in disposal of aggregated proteins that might have toxic effects on neurons (38, 39). In fact, the neuropathological effects of Aβ in a mouse model of AD were ameliorated by enhancing autophagy via overexpression of the autophagy protein beclin 1 (39). In a study by Cohen et al., breeding of a mouse model of AD to a mouse model with targeted disruption of the IGF-1 receptor demonstrated that reduced IGF-1 signaling blunted and delayed the toxic effect of Aβ accumulation (40). Although this could be attributed in part to sequestration of soluble Aβ oligomers into dense aggregates of lower toxicity, it is well established that IGF-1 signaling inhibits autophagy and therefore that these mice would likely have enhanced autophagy. Thus, based on the current literature, autophagy may be increased in AD, but the load of oligomers may be too great to avoid toxic Aβ accumulation.
Other disorders associated with increased protein aggregates include Parkinson's Disease and Huntington's Chorea. Parkinson's Disease is associated with the presence of protein aggregates in the form of “Lewy Bodies”, which contain a number of proteins including one or more of alpha-synuclein, ubiquitin, neurofilament protein, alpha B crystallin and tau protein. Interestingly, a number of other disorders manifested as dementia are also associated with the presence of Lewy Bodies in neurons—these include Alzheimer's Disease, Pick's Disease, corticobasal atrophy, multiple system atrophy, and so-called “dementia with Lewy Bodies” or “DLB”. Huntington's Chorea is associated with aggregates of huntingtin protein containing a mutation that results in long tracts of polyglutamine (“polyQ”) which result in improper protein processing and aggregate formation.
Carbamazepine (“CBZ”; also known as Tegretol®, Carbatrol, and Equetro), is a drug that has been used for many years as an anticonvulsant in the treatment of epilepsy and as a specific analgesic for treatment of trigeminal neuralgia. It is believed to act by reducing post-synaptic responses and blocking post-tetanic potentiation in the nervous system. CBZ is known to increase hepatic cytochrome P450 activity and thereby affect the clearance of other pharmaceuticals eliminated through that system. It is metabolized in the liver (see Prescribing Information from Novartis Pharmaceuticals).
Oxcarbazepine (“OBZ”, also known as Trileptal®) is, like CBZ, a drug used in the treatment of seizures and trigeminal neuralgia; in addition, it is used as a mood stabilizer. Unlike CBZ, neither OBZ nor its monohydroxy derivative induce hepatic oxidative metabolism (with the possible exception of P450IIIA isozyme (58).